In a typical cellular system, also referred to as a wireless communications network, communication network or network, wireless terminals, also known as mobile stations and/or User Equipment units (UEs) communicate with a radio network node in a Radio Access Networks (RAN). The UE is a mobile terminal by which a subscriber may access services offered by an operator's Core Network (CN) and services outside operator's network to which the operator's RAN and CN provide access The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called an evolved NodeB (eNB), eNodeB, B node or base station. The base station communicates over the air interface operating on radio frequencies with the user equipment(s) within range of the base station. The RAN may implement different Radio Access Technologies (RAT), such as Long Term Evolution (LTE), Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), (Worldwide Interoperability for Microwave Access (WiMAX) etc.
FIG. 1 is a block diagram illustrating embodiments of the architecture of a communication network 100 comprising a home network 100h and a visited network 100v. The home network 100h refers to the network being registered as the home of a user equipment (not shown). A user equipment may roam between for example the home network 100h and the visited network 100v. The network 100 comprises Evolved-UMTS Terrestrial Radio Access Network (E-UTRAN) 101 on the access side and Evolved Packet Core (EPC) on the core side. E-UTRAN 101 is the air interface of LTE. The E-UTRAN 101 comprises base stations, such as e.g. eNodeBs (not shown), which interfaces with user equipment(s) (not shown). The continuous line represents user plane and the dotted line represents the control plane. For illustration purposes only, the home network 100h and the visited network 100v are separated by a dotted line, which is different from the dotted line representing the control plane.
The home network 100h comprises a Home Subscriber Server (HSS) 103, a home PDN GW (H-PDN GW) 105h and a home Policy charging and rules function (H-PCRF) 107h. The H-PDN GW 105h is connected to a packet data network providing operator Internet Protocol (IP) Services 110 such as IP Multimedia Subsystem (IMS), PSS etc. IMS is an architectural framework for delivering Internet Protocol (IP) multimedia services. The HSS 103 is a database comprising subscription data and authentication data. The HSS 103 is connected to a mobile management entity (MME) 113 in the visited network 100v via an S6a interface. The MME 113 is for example responsible for choosing the appropriate PDN GW 105 for a user equipment data session. The H-PDN GW 105h is the node in the network 100 that allocates and reserves IP addresses to user equipments currently registered to the network 100. The H-PDN GW 105h also takes care of policy and charging enforcement, and receives the policies from the H-PCRF 107h. The visited network 100v comprises a Serving GateWay (SGW) 115 which routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies. The SGW 115 in the visited network 100v is connected to the H-PDN GW 105h via an S8 interface. The S8 interface is an inter-public land mobile network (PLMN) reference point providing user and control plane between the SGW 115 in the visited network 100v and the H-PDN GW 105h in the home network 100h. 
The visited network 100v further comprises a visited PDN GW (V-PDN GW) 105v. The V-PDN GW 105v also takes care of policy and charging enforcement, and receives the policies from a visited PCRF (V-PCRF) 107v. The interface between the H-PCRF 107h and the V-PCRF 107v is called S9. The S9 interface provides transfer of policy and charging control information between the H-PCRF 107h and the V-PCRF 107v in order to support local breakout function.
The Long Term Evolution (LTE)/Evolved Packet Core (EPC) architecture supports Packet Switched (PS) roaming, as illustrated in FIG. 1, and is similar to the one deployed for Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access, (WCDMA).
For LTE/EPC a Packet Data Network Gateway in the home network 100h, i.e. a H-PDN GW 105h, is usually selected, although the third Generation Partnership Project (3GPP) specifications also allow a PDN GW in the visited network 100v, i.e. V-PDN GW 105v to be selected.
When an operator decides to add support for IMS services such as e.g. voice services, additional network elements/nodes are required compared to what is illustrated in FIG. 1. The additional network elements are illustrated in the high-level IMS Voice Roaming Architecture of an embodiment of a communication network 200 shown in FIG. 2. The continuos line depicts the media plane, and the dotted line depicts the control plane.
Some of the elements in the network 200 shown in FIG. 2 are the same as the elements shown in the network 100 in FIG. 1. For the sake of simplicity, a description of these elements are not repeated here with reference to FIG. 2. Only the additional elements needed for the IMS services are described in detail with reference to FIG. 2.
Internetwork Packet Exchange (IPX) 202 is the Open Systems interconnection (OSI)-model Network layer protocol located between the home network 100h and the visited network 100v. 
The IMS 217 defines a generic architecture for offering Voice over IP (VoIP) and multimedia services. It is an architectural framework for interoperability and roaming, and it provides bearer control, charging and security. IMS 217 comprises an Application Server (AS) 220 and a Serving-Call Session Control Function (S-CSCF) 222. The AS 220 host and execute services, and interface with the S-CSCF 222 using Session Initiation Protocol (SIP). The S-CSCF 222 is the central node of the signaling plane, and is always located in the home network 100h. 
The visited network 100v comprises a Proxy-CSCF (P-CSCF) 225. In some embodiments, the P-CSCF 225 may be in the home network 100h instead of in the visited network 100v, i.e. when the visited network is not IMS compliant yet. The interface between the S-CSCF 222 and the P-CSCF 225 is called Mw, and it is used to exchange messages between CSCFs.
The visited network 100v further comprises an IMS-Access GateWay (IMS-AGW) 228.
This network element provides an interface between the radio network, i.e. access network, and the IP Interconnect Network 230.
Note that a V-PDN GW 105v in the visited network 100v is assumed. The advantage compared to a H-PDN GW 105h in the home network 100h is for example to keep the latency low in different roaming scenarios and that the visited network 100v may be service aware.
However, there are other IMS services than voice, and such service may use a home routed Access Point Name (APN), in case the visited network 100v doesn't support the IMS APN.
To enable the usage of a V-PDN GW 105v in the visited network 100v, an APN that points to a visited network V-PDN GW 105v is required.
If a user equipment 233, that uses the IMS APN for voice services in its home network 100h, visits a network that doesn't support IMS voice, the user equipment 233 shall still use the IMS APN to negotiate any supplementary service for IMS voice service settings.
However, there are mechanisms defined to tell the user equipment 233 if IMS voice is supported in the visited network 100v. 
The serving network, i.e. serving public land mobile network (PLMN), shall send an indication toward the user equipment 233 during the Attach procedure and Tracking Area Update procedures if an IMS voice over Packet Switched (PS) session is supported. The serving network uses this indicator to indicate to the user equipment 233 whether it may expect a successful IMS voice over PS session. A user equipment 233 with “IMS voice over PS” voice capability should take this indication into account when establishing voice over PS sessions as well as when determining whether to deactivate the special handling of Idle mode Signaling Reduction (ISR) locally.
The serving network provides this indication based e.g. on local policy, home network 100h, the Single Radio Voice Call Continuity (SRVCC) capability of the network 200 and user equipment 233 and/or extends of E-UTRAN/UTRAN coverage. The serving network indicates to the user equipment 233 that the user equipment 233 may expect a successful IMS voice over PS session only if the MME 113 is configured to know that the serving network has a roaming agreement for IMS voice with the home network 100h of the user equipment 233. This indication is per Tracking Area Identity (TAI) list.
For Voice over LTE roaming to work, a “well-known” Access Point Name (APN) used for IMS services has been defined. The APN name must be “IMS”, which is also the APN Network Identifier part of the full APN. The APN Operator Identifier part of the full APN depends on the PLMN who's PDN GW 105 the user equipment 233 is anchored to.
The IMS APN should be provisioned as the Default APN for the IMS subscriber. When provisioning the IMS APN, the following subscription details have to be taken into account:                All Packet Oriented Services must not be barred        Packet Oriented Services from access points that are within the roamed to visited network 100v must not be barred        Visited network 100v Address must be allowed for the IMS APN        EPS subscribed Quality of Service (QoS) profile QoS Class Identifier (QCI) must be set to five for the APN        The APN must be named “IMS”        
If the user equipment 233 provides the IMS APN to the network, then the gateway selection logic follows the “An APN was sent by the MS” procedures. The UE should not provide the APN Operator Identifier so that the expected gateway selection logic would be the same as in the case where the network provided the Default APN.
If some operators are using the IMS APN without having IMS roaming agreement in place with all other operators, roaming users may get the IMS APN resolved to a PDN GW 105 in a visited network 100v without IMS roaming agreement. The “IMS voice over PS Session Supported Indication” will help the user equipment 233 to decide if IMS voice may be used or not, but it doesn't state if the IMS APN may be used for supplementary services interrogation or for example for Rich Communication Suite (RCS). The result will be that the user equipment 233 gets the IMS APN resolved to the local PDN GW 105, and the local P-CSCF 107. But, since there is no IMS roaming agreement, only an LTE roaming agreement, any IMS traffic will not be forwarded to the home IMS network 100h, and the roaming end user may not get any IMS services.
A home operator may avoid roaming users to get a local PDN GW 105 by setting the “VPLMN Address Allowed” flag in HSS 103 to stop visited services, but this will stop all roaming for the user on this APN. In a scenario where some operators have IMS roaming agreements and some haven't, this flag may not be used. For example if operators A and B have roaming agreements with each other, and C and D, but not A and C. The same problem arises in 2G and 3G networks.
A computer network ties a number of computers, terminals and wireless devices together for exchange of information. These computers, terminals and wireless devices are also called nodes of the network.